Signal correction techniques

ABSTRACT

Errors present in the output of an amplifier ( 10 ) are compared with the amplifier input in a control unit ( 24 ) to derive corrected coefficients for a look-up table used by predistorter ( 12 ) to eliminate distortion in the output of the amplifier ( 10 ). Depending on the format of the predistorter ( 12 ), at least some of the coefficients are corrected using measured errors that have been rendered independent of the input signal&#39;s amplitude.

[0001] The invention relates to methods of, and apparatus for,controlling distortion counteracting equipment such as a predistorter.

[0002] In a case where signal handling equipment distorts a signal uponwhich it operates, it is known to use a lineariser to reduce distortionin the output signal of the equipment. It is known to performlinearisation adaptively by monitoring errors in the output signal andusing information on these errors to adjust the linearisation to reducethe errors as far as possible. Typically, predistorters are used tolinearise amplifiers.

[0003] One aim of the invention is to provide improvements in the mannerin which distortion counteracting equipment, such as a predistorter, iscontrolled.

[0004] According to one aspect, the invention provides apparatus foradapting distortion counteracting equipment, wherein said counteractingequipment employs a group of coefficients to adjust a consequentialsignal in order to ameliorate distortion in an output signal produced bysignal handling equipment in response to an input signal, the apparatuscomprising error measuring means for measuring errors in the outputsignal, modifying means for modifying measured errors to render themless dependent on the input signal's amplitude and correcting means forchanging said coefficients by amounts dependent on modified errors.

[0005] The invention also consists in a method of adapting a distortioncounteracting equipment, wherein said counteracting equipment employs agroup of coefficients to adjust a consequential signal in order toameliorate distortion in an output signal produced by signal handlingequipment in response to an input signal, the method comprisingmeasuring errors in the output signal, modifying measured errors torender them less dependent on the input signal's amplitude and changingsaid coefficients by amounts dependent on modified errors.

[0006] In one embodiment, the consequential signal is the input signalto the signal handling equipment and the counteracting equipment is apredistorter.

[0007] Thus, the invention improves the ability of distortioncounteracting equipment to converge to the best values for thecoefficients to optimise the elimination of distortion in the outputsignal. This is achieved by reducing the dependence of the changes tothe coefficients on the input signal amplitude (which can vary greatly,e.g. in the case of CDMA signals), i.e. by reducing the dependence ofthe coefficients' loop gains on the input signal's amplitude. This meansthat the coefficients converge equally quickly.

[0008] In one embodiment, the coefficients are indexed by a function ofthe input signal's amplitude such that the input signal's amplitudeselects the coefficient or coefficients that are used in the distortionamelioration process at any given time. Preferably, the coefficients arearranged in one or more look-up tables indexed by functions of the inputsignal's amplitude.

[0009] In one embodiment, the measured errors are indexed according to afunction of the input signal's amplitude, i.e. the measured errors aretabulated against a function of the input signal's amplitude. Forexample, measured errors can be obtained for each of a series of valuesof the function of the input signal's amplitude. Where the coefficientsare also indexed against the same function of the input signal'samplitude, it is possible to arrange the adaption process such that ameasured error is used in the adaption of the coefficient sharing thesame index value.

[0010] Where a function, f(a), of the input signal's amplitude, a, isused to index coefficients or errors, the function may be simple, suchas f(a)=k.a, or more complex, such as f(a)=k.a² (here, k is an arbitraryconstant). In the simplest case, f(a)=a.

[0011] In one embodiment, errors in a particular parameter are measuredand these errors are used to adjust coefficients which act on the samesignal parameter.

[0012] In one embodiment, the coefficients are segregated intosub-groups, each sub-group for altering a different parameter of theconsequential signal. The errors used to adapt coefficients in asub-group may be errors in the parameter to which the sub-grouppertains. For example, where the coefficients fall into amplitude andphase sub-groups, the coefficients in the sub-groups are adapted on thebasis of amplitude and phase errors respectively. Where the coefficientsfall into sub-groups for orthogonal components, errors in eachorthogonal component are used to adapt the coefficients for therespective component.

[0013] In one embodiment, the error values are derived over a period oftime to reduce the effects of noise on the error values. For example,the error values could be accumulated and averaged over a period oftime. The error values may be processed to remove the affects of noise,e.g. by filtering the errors or by fitting them to a curve. Similarly,after adaption, the coefficients may be processed to remove the affectsof noise, e.g. by filtering the adapted coefficients or by fitting themto a curve.

[0014] Preferably, the modified errors are substantially independent ofthe input signal's amplitude.

[0015] In one embodiment, at least some measured errors are modified bydividing them by the input signal's amplitude to make them substantiallyindependent of the input signal's amplitude. Where a measured error isdivided by the input signal's amplitude and the measured errors areindexed by a function of the input signal's amplitude, the measurederror can be divided by the amplitude value which indexes that measurederror.

[0016] In one embodiment, the changes to the coefficients areproportional to modified errors. Preferably, the changes to thecoefficients are fractions of the modified errors. Advantageously, thefractions can be changed to optimise the adaption rate of thecoefficients. For example, the size of the fractions may depend on thesize of the errors such that the smaller the error, the smaller thefraction and the larger the error, the larger the fraction.

[0017] The invention also consists in a system comprising distortioncounteracting equipment for ameliorating signal distortion in signalhandling equipment and comprising adaption apparatus as described abovefor adapting the counteracting equipment.

[0018] The invention has been stated above in terms of methods ofadapting distortion counteracting equipment; the invention extends alsoto programs for performing such methods. These programs could be held ina suitable data store such as a disk or memory.

[0019] In a preferred embodiment, the signal handling equipment isequipment for amplifying the input signal to create the output signal,comprising one or more amplifiers.

[0020] In the preferred embodiments, the distortion counteractingequipment is a lineariser. In a particularly preferred embodiment, thelineariser is a predistorter and the coefficients are for predistortingthe input signal to the signal handling equipment. In anotherembodiment, the lineariser is a feed-forward lineariser in which theinput signal is sensed, modified by the coefficients and combined withthe amplifier output to give a linearising effect.

[0021] By way of example only, certain embodiments of the invention willnow be described with reference to the accompanying figures, in which:

[0022]FIG. 1 is a block diagram of a predistorter arrangement operatingon an amplifier;

[0023]FIG. 2 is a signal space diagram illustrating the discrepancybetween ideal and actual output signals for the amplifier undergoinglinearisation in FIG. 1;

[0024]FIG. 3 is a flowchart illustrating a routine for adapting apredistorter which operates according to a quadrature signal format; and

[0025]FIG. 4 is a flowchart illustrating a routine for adapting apredistorter which operates according to an amplitude and phase signalformat.

[0026]FIG. 1 illustrates an arrangement for predistorting a radiofrequency input signal RF₁ for an amplifier 10. The input signal RF₁ ispredistorted in the digital domain using digital predistorter 12 toproduce a digitally predistorted input signal S₃ for amplifier 10. Adownconverter 14 reduces the frequency of the RF input signal RF₁ to afrequency supported by the sampling rate at which the predistorter 12operates. The downconverted input signal is converted to the digitaldomain by analogue to digital converter (ADC) 16. The predistorted inputsignal S₃ is returned to the analogue domain by digital to analogueconverter (DAC) 18 and upconverted to a desired frequency by upconverter20 prior to being supplied as an input to amplifier 10.

[0027] The predistorter 12 can be adapted on the basis of feedback fromthe output RF₂ of amplifier 10. Discrepancies between the actual outputRF₂ of amplifier 10 and its ideal output are used to adapt thepredistortion of the amplifier input signal. The feedback signal isdownconverted at downconverter 22 to a signal which is supported by thesampling rate of the digital aspect of the system, comprisingpredistorter 12 and control unit 24. The output of downconverter 22 isdigitised by ADC 26 and supplied to control unit 24 as a feedback signalS₂. The control unit 24 uses the digitised feedback signal S₂ and thedigitised input signal S₁ (from ADC 16) to adapt the predistortion.

[0028] If the original frequency of the input signal RF₁ is notincompatible with the sampling rate of predistorter 12, thendownconverter 14 is not needed. If downconverter 14 is absent, thenupconverter 20 is not required unless it is desired to shift thepredistorted amplifier input signal to a frequency different from thatof the original input signal RF₁. If the sampling rate of the digitalaspect of the system supports the frequency of RF₂ then feedbackdownconverter 22 is not needed. The digitisation performed by ADCs 16and 26 is performed in such a way as to preserve the amplitude and phaseinformation in the signals being operated upon. For example, if theinput to one of the ADCs is at baseband then the signal must be splitinto two paths with one path being in phase-quadrature to the secondpath with both paths then being digitised separately.

[0029] In this embodiment, the amplitude and phase of signal Si aremodified in the predistorter 12 by the action of look-up tables (LUTs).This can be achieved by using a gain look-up table L_(G) and a phaselook-up table L_(P), each indexed by a function f of the amplitude A₁ ofsignal S₁. Thus, when signal S₁ has amplitude A₁, value L_(G) (f(A₁)) isretrieved from look-up table L_(G), value L_(P) (f(A₁)) is retrievedfrom look-up table L_(P) and these values are then used to predistortthe signal S₁ according to the following equation:

S ₃ =L _(G)(f(A ₁)).S ₁ .e ^(jL) _(^(p)) ^((f(A) _(¹) ⁾⁾, where S ₁ =A ₁.e ^(ja)

[0030] Alternatively, predistorter 12 can contain an in-phase LUT L₁ anda quadrature phase LUT L_(Q). Again, these look-up tables are indexed bya function f of the amplitude A₁ of signal S₁ in order to provide valuesL₁ (f(A₁)) and L_(Q) (f(A₁)), respectively. The retrieved LUT values arethen used to predistort the input signal S₁ in quadrature formataccording to the following equation:

S ₃ =L ₁(f(A ₁)).S ₁ +j.L _(Q)(f(A ₁)).S ₁, where S ₁ =A ₁ .e ^(ja)

[0031] The function f of the input signal amplitude A₁ that is used toindex the look-up tables can take any one of a large number of forms.The best form for f in any particular case is dependent on thecharacteristics of the modulation on the input signal RF₁. For example,the two most obvious forms are f(A₁)=C.A₁ and f(A₁)=C.A₁ ² where C is anarbitrary constant which can be 1.

[0032] To adapt the predistortion in response to errors in the output ofamplifier 10, the control unit 24 compares the signals S₁ and S₂ togenerate updated LUT values which are transferred to the LUTs in thepredistorter 12. Prior to comparison, the signal vectors S₁ and S₂ are,within control unit 24, time aligned (for example by digitally delayingS₁) so as to remove any relative delay difference between them and phasealigned (for example by digitally adjusting the phase of S₁ or S₂) toeliminate any phase offset between them. These alignment processesenable an accurate comparison of S₁ and S₂ to allow the error betweenthem and its relationship to the input signal amplitude A₁ to bedetermined independently of the modulation frequency.

[0033] The ideal or target output signal vector S₂, of amplifier 10 isdefined as a linear function of the input, i.e. S₂=G.S₁ where G is aconstant. An error signal vector or error vector S_(e) is defined as thedifference between the measured output signal vector and the targetoutput signal vector, i.e. S_(e=)S₂−S₂,. The error vector Se is thendescribed in terms of amplitude A_(e) and phase P_(e) components or interms of two orthogonal vectors I_(e) and Q_(e) depending on whether thepredistorter LUTs are in the amplitude and phase format or thequadrature format respectively. The relationship between the vectorsS_(2, S) ₂, and S is shown in the complex signal space diagram of FIG.2, which also illustrates the components A₁, PC, IC and Q_(e) of S_(e)(note that I_(e) and Q_(e) are not aligned with the I and Q axes ofsignal space but rather I_(e) is parallel to S_(2t)).

[0034] Thus, as time progresses, various values of S_(e) (in theappropriate one of the A_(e), P_(e) and I_(e), Q_(e) formats) arerecorded. Since signal S₁ has a time varying amplitude A₁, the recordederror signals S_(e) are obtained for various input signal amplitudes A₁.The error signals S_(e) obtained are tabulated against bins or ranges ofthe function of the input signal amplitude f(A₁) that is used to indexthe predistorter LUTs. Between predistorter updates, all error signalsS_(e) falling within the same bin are accumulated and averaged to derivea mean error for that bin. Notionally therefore, the table created bytabulating the error signals S_(e) against f(A₁) is a table of meanerrors indexed by f(A₁). The table of mean errors versus f(A₁) can befiltered or curve-fitted to remove the effects of noise. The table ofmean errors is then used to adjust the predistorter LUT values.

[0035] Where the predistorter operates amplitude and phase LUTs, eachvalue in the LUTs is adjusted according to the appropriate one of thetwo following equations:${L_{G{(n)}}\left( {f\left( A_{1} \right)} \right)} = {{L_{G{({n - 1})}}\left( {f\left( A_{1} \right)} \right)} - {r \cdot {A_{e{({n - 1})}}\left( {f\left( A_{1} \right)} \right)} \cdot \frac{1}{A_{1}}}}$

 L _(P(n))(f/A ₁))=L _(P(n−1))(f(A ₁))−r.P _(e(n−1))(f/A ₁))

[0036] Alternatively, where the predistorter operates with in-phase andquadrature-phase LUTs, the look-up table values are adjusted using thefollowing equations: $\begin{matrix}{{L_{I{(n)}}\left( {f\left( A_{1} \right)} \right)} = {{L_{I{({n - 1})}}\left( {f\left( A_{1} \right)} \right)} - {r \cdot {I_{e{({n - 1})}}\left( {f\left( A_{1} \right)} \right)} \cdot \frac{1}{A_{1}}}}} \\{{L_{Q{(n)}}\left( {f\left( A_{1} \right)} \right)} = {{L_{Q{({n - 1})}}\left( {f\left( A_{1} \right)} \right)} - {r \cdot {Q_{e{({n - 1})}}\left( {f\left( A_{1} \right)} \right)} \cdot \frac{1}{A_{1}}}}}\end{matrix}$

[0037] In the four preceding equations, n−1 and n denote that thecurrent LUT values (n−1) are used to produce the new LUT values (n).

[0038] The two pairs of equations above each describe a feedback controlloop with the subtracted terms representing the feedback. The loop gainfor each entry in the LUTs is defined as the ratio of the change in thefeedback term to the change in the LUT value, i.e.$\frac{\Delta \quad {feedback}\quad {term}}{L_{x{(n)}} - L_{x{({n - 1})}}}$

[0039] where x is G, P, I or Q as appropriate. In the case of each ofthe four feedback control loop equations above, it can be shown that theloop gain is approximately proportional to r and is independent of theinput signal amplitude A₁ (provided that the effect of the amplifiernon-linearity is ignored). The independence of the loop gain from A₁ isdue to the term $\frac{1}{A_{1}}$

[0040] which appears in the feedback equations except that for L_(P(n)).Using the above equations, the LUT values are updated for all values ofthe index f(A₁). The revised LUT values can be filtered or curve-fittedto remove the effects of noise if desired.

[0041] The control unit 24 is able to vary the size of the loop gain byvarying the size of r in response to the magnitude of the error vectorS_(e). If the distortion is judged to be high then the loop gain is maderelatively large by setting r to a relatively large value so that theLUT values converge quickly to a solution which minimises the distortionin signal S₂. On the other hand, if the distortion in S₂ is low, thenthe LUT values are already approximately correct and a relatively smallloop gain is selected by setting r to a relatively small value so thatthe effects of system noise and spurious signals on the shape of thelook-up tables are minimised. The preferred method for setting r is touse a mathematical function to generate a number based on the mean errortable information. For example, a sum of squares calculation may beperformed on the mean errors in the table. A large result implies alarge amount of distortion at the amplifier output and hence that alarge loop gain is required to arrive at a solution for the LUT values.A small result implies that there is only a small amount of distortionin the output of the amplifier and hence that a small loop gain isrequired, giving slower convergence of the LUT values.

[0042]FIG. 3 is a flowchart which further explains the process ofupdating predistorter LUT values in the case where in-phase andquadrature LUTs are used.

[0043]FIG. 4 is a flowchart further explaining the process of updatingpredistorter values in the case where amplitude and phase LUTs are used.

[0044] In the embodiments described above, the predistorters are vectorpredistorters which are capable of reducing both AM (amplitudemodulation) to AM distortion and AM to PM (phase modulation) distortion.It will be apparent to the skilled person that the predistorter could bea scalar predistorter which counteracts only either AM to AM or AM to PMdistortion by providing only a gain or phase LUT respectively with theresult that only gain or phase errors need to be tabulated forsubsequently adjusting the LUT values.

1. Apparatus for adapting distortion counteracting equipment, whereinsaid counteracting equipment employs a group of coefficients to adjust aconsequential signal in order to ameliorate distortion in an outputsignal produced by signal handling equipment in response to an inputsignal, the apparatus comprising error measuring means for measuringerrors in the output signal, modifying means for modifying at least somemeasured errors to render them less dependent on the input signal'samplitude and correcting means for changing coefficients by amountsdependent on modified errors. 2-26 (canceled)